1. Field of the Invention
The present invention relates generally to metallized film used in capacitors and, more particularly, to a segmented metallized film having fuses that protect a capacitor in the event of a short circuit.
2. Background of the Invention
Metallized film capacitors are typically made of two tightly wound sheets wrapped around a core. Each sheet is composed of a dielectric film having a metallized layer on one face of the film. The metallized layer extends to one edge of the face to provide a metallized edge. The metallized layer stops short of the opposing edge of the face to provide an unmetallized edge. The unmetallized edges of the two sheets are placed opposite to each other when the sheets are stacked and wound together, such that only one metallized edge is available for connecting to a lead at each end of the rolled capacitor. Each end is sprayed with a conductive metal that bonds with the sheet having a metallized edge at that end. Leads are then attached to each sprayed end to form the capacitor electrodes. The rolled capacitor is then placed in a housing and impregnated with a dielectric fluid or encapsulated in a resin.
FIG. 1 illustrates a conventional metallized film capacitor 100. As shown, capacitor 100 includes two metallized films 102 placed on top of each other and wound around a core (not shown) to form a wound section 104. Each metallized film 102 includes a dielectric film 106 that is coated on one side with a metallized layer 108. The metallized layers 108 of the dielectric films 106 are separated by the unmetallized side of at least one of the dielectric films 106.
Each metallized film 102 includes a metallized edge 110 and an unmetallized area 112. The metallized edges I 10 of the metallized films 102 are oppositely positioned during winding of the metallized films 102. Accordingly, the unmetallized areas 112 are also oppositely positioned. In this manner, after tightly winding metallized films 102 around the core, only one metallized edge 110 is available for connecting to a lead at each end of wound section 104. The ends of wound section 104 are covered with a conductive metallic spray 114. Leads 116 are connected to conductive metallic spray 114 to form electrodes 118.
For purposes of defining the present invention, the direetion in which metallized films 102 are wound is referred to herein as the machine direction and is represented by arrow 120.
Metallized film capacitors are non-polar and xe2x80x9cself-clearingxe2x80x9d (which is also referred to as xe2x80x9cself-protectedxe2x80x9d or xe2x80x9cself-healingxe2x80x9d). Thus, when a metallized film capacitor is exposed to, for example, an excessive voltage, a short circuit develops through the dielectric film, between the metallized layers (i.e., plates) of the capacitor. The current from the short circuit vaporizes small areas of the metallized layer near the short circuit. The vaporization of the metallized layer opens the short circuit, after which the capacitor returns to normal function, usually with only a small reduction in microfarads. The typical xe2x80x9cend of lifexe2x80x9d of these types of capacitors are xe2x80x9copensxe2x80x9d caused by the accumulated, gradual loss of dielectric film metallized electrodes from repeated xe2x80x9cself-clearingxe2x80x9d or activation of protective devices (e.g., interrupters).
To minimize the loss in capacitance resulting from the loss of a portion of the metallization layer during a short circuit, manufacturers have developed xe2x80x9csegmentedxe2x80x9d or xe2x80x9cpatternedxe2x80x9d metallization films. These segmented metallization films include unmetallized margin areas that divide the metallization layer into multiple metal segments interlinked by small fuses. If a short circuit occurs within a segment, the fuses that interlink that segment to adjacent segments open and remove the segment from the capacitor, thereby avoiding a catastrophic failure of the capacitor and the circuit in which the capacitor is used.
FIGS. 2A-2C and 3A-3C illustrate two examples of conventional self-protected metallized film patterns. FIGS. 2A-2C illustrate the xe2x80x9c13Mxe2x80x9d pattern used by Bollore of Ergue-Gaberic, France to make self-protected metallized polypropylene sections. FIGS. 3A-3C illustrate a segmented film pattern manufactured by Steinerfilm Inc. of Williamstown, Massachusetts.
FIG. 2A shows a segmented metallized film 200 made of a dielectric film 202 coated with a metallized layer 204. Metallized layer 204 covers most of dielectric film 202, including a metallized edge 206. Opposite the metallized edge 206, metallized layer 204 stops short of an unmetallized edge 208 to provide an uncoated area 210 (which is approximately 2.50 mm wide). Metallized layer 204 includes a lattice pattern of margin areas 212 that divide metallized layer 204 into multiple segments 214. Approximately 6.0 mm from metallized edge 206, lattice pattern 212 includes an edge margin area 216 that contains edge fuses 218. The width of edge margin area 216 is approximately 0.2 mm. The length of edge fuses 218 is approximately 0.6 mm.
FIG. 2B illustrates an enlarged view of a segment 214 of metallized film 200. As shown, the margin areas of lattice pattern 212 include a segment fuse 220 along each length of margin area that encloses segment 214. Segment fuses 220 interconnect adjacent segments 214. In this manner, when a faulting segment develops a short circuit, the segment fuses that interconnect the faulting segment to adjacent segments break, thereby isolating the faulting segment.
FIG. 2C illustrates in greater detail a fuse 220 and the margin areas of lattice pattern 212 in the area of fuse 220. As shown, the margin areas of lattice pattern 212 on either side of fuse 220 are squared. The width 224 of the margin areas of lattice pattern 212 is approximately 0.15 mm. The length of fuse 220 is approximately 0.30 mm.
FIG. 3A shows another example of a segmented metallized film 300 made of a dielectric film 302 coated with a metallized layer 304. Metallized layer 304 covers most of dielectric film 302, including a metallized edge 306. Opposite the metallized edge 306, metallized layer 304 stops short of an unmetallized edge 308 to provide an uncoated area 310 (which is approximately 2.50 mm wide). Metallized layer 304 includes a lattice pattern of margin areas 312 that divide metallized layer 304 into multiple segments 314. Approximately 4.7 mm from metallized edge 306, lattice pattern 312 includes an edge margin area 316 that contains edge fuses 318. The width of edge margin area 316 is approximately 0.33 mm. The length of edge fuses 318 is approximately 1.5 mm.
FIG. 3B illustrates an enlarged view of a segment 314 of metallized film 300. As shown, the margin areas of lattice pattern 312 include a segment fuse 320 along each length of margin area that encloses segment 314. Segment fuses 320 interconnect adjacent segments 314. In this manner, when a faulting segment develops a short circuit, the segment fuses that interconnect the faulting segment to adjacent segments break, thereby isolating the faulting segment.
FIG. 3C illustrates in greater detail a fuse 320 and the margin areas of lattice pattern 312 in the area of fuse 320. As shown, the margin areas of lattice pattern 312 on either side of fuse 320 are rounded. The width 324 of the margin areas of lattice pattern 312 is approximately 0.33 mm. The length of fuse 320 is approximately 0.24 mm.
Comparing film 200 of FIGS. 2A-2C to film 300 of FIGS. 3A-3C, film 300 has fewer and larger segments, with wider margin areas. That is, the width 322 of FIG. 3B (approximately 9.4 mm) is greater than width 222 of FIG. 2B (approximately 7 mm). In terms of the width of the margin areas that define the lattice pattern, segmented metallized film 300 has wider margin areas. That is, the width 324 of FIG. 3C (approximately 0.33 mm) is greater than the width 224 of FIG. 2C (approximately 0.15 mm). In either case, however, there is a substantial amount of unmetallized area due to the lattice pattern of the margin areas. These patterns therefore reduce the active area of metallization an average of 8-15%, representing an inefficient and costly use of the metallized film.
In addition to this loss in active area, the proximity of the edge margins 216 and 316 to the metallized edges 206 and 306, respectively, causes some of the active electrode area to remain in contact with energy sources even after a fault occurs. In the area near metallized edges 206 and 306, metallized layers 204 and 304, respectively, tend to be heavier and less uniformly applied. Consequently, the first line of fuses in the edge margins 216 and 316 tends to be in an area of heavy metallization (also referred to as the xe2x80x9cheavy edgexe2x80x9d) and, even when open circuited, can leave some active electrode. Although these patterns still typically fail safely at their ends of life cycle, it would be more desirable to completely remove the active electrode from energy sources when a fault occurs.
The present invention provides a segmented metallized film having fuses in approximately the middle of the film sheet, outside the area of heavy metallization. In comparison to the films of the prior art, the present invention uses less margin area and therefore minimizes the uncoated area of the metallized film. For example, instead of the prior art""s 8-15% loss in active area, the present invention can limit the loss due to margin areas to as little as a 2-3%. While providing this more efficient use of the active electrode area, the segmented metallized film of the present invention still allows a capacitor to fail safely at the end of its life and to meet commonly accepted performance criteria under normal operating conditions.
An embodiment of the present invention provides a segmented metallized film having a dielectric film, a metallized layer, and at least one fuse disposed within each segment along a center margin of the metallized layer. The metallized layer is on a portion of the dielectric film such that the dielectric film has a metallized edge, an unmetallized edge, and an uncoated area between the metallized layer and the unmetallized edge. The center margin is substantially parallel to the machine direction of the film. The center margin is disposed in the metallized layer approximately equidistant to the metallized edge and to the unmetallized edge. The metallized layer has a plurality of transverse margins running from the center margin to the uncoated area. The center margin and the plurality of transverse margins define a plurality of segments. At least one fuse is disposed in the center margin within each segment of the plurality of segments.
In an embodiment of the present invention, the center margin is offset from the centerline between the metallized and unmetallized edges so that when the fuses open the circuit, all of the active area is removed from the external circuit. In addition, offsetting the center margins of two metallized films of a capacitor separates the fuses so that any heat generated by I2R losses in the fuse area is not concentrated in the center of the section.
The fuse arrangement of the present invention enables capacitors to fail safely, while at the same time, maximizing active area. As a comparison, capacitors made with the conventional patterns shown in FIGS. 2A-2C and 3A-3C generally show only the first and second lines of fuses open circuiting, when tested under the fault conditions specified in UL 810. The remaining patterns and fuses are removed from the circuit when the first line of fuses operates. In contrast, with the present invention, only the segmentation is needed to bring the capacitor safely toward an open circuit. Thus, instead of the two lines of fuses used in the prior art, which wastes the active area, the present invention provides only one line of fuses, thereby maximizing the use of the active area.